The earth is       years old.

A．four thousands six hundreds

B．four thousand six hundred

C．four thousand and six hundred million

D．four thousand six hundred millions

两块板厚为8mm，宽为500mm的钢板对接在一起，两端受450000N的拉力，该钢焊缝许用拉应力[σt′]为132MPa，试校核其强度。